This invention is related to mounted optics for laser processing of materials, particularly where the lasers are controlled by a closed-loop system using feedback data from a “control” beam that measures or illuminates, but does not process, the material. This invention is particularly applicable to ablation laser systems controlled by autofocus mechanisms, but can also be useful in laser cutting, welding, or annealing systems that use control beams to track and correct beam-pointing errors or to illuminate markers or fiducials.
Laser processing tools are widely used in many industries. Factories mass-producing high-precision components or products often have tens or hundreds of working lasers operating simultaneously. Processing lasers generally produce much more powerful beams than most lasers used in other common applications such as displays, data recorders and readers, and printers. High-power lasers often have shorter operating lifetimes and more reliability problems than their lower-power counterparts, because high-power lasers routinely exert more thermal, electrical, or other wear and tear on both internal and external components, and they can quickly and catastrophically damage their internal and external optics and mechanics if even a small amount of contaminant that absorbs the laser wavelength—such as a tiny speck of dust or a light film of outgassing residue—enters the beam path on or near a surface. The more processing lasers a factory operates simultaneously, the greater the probability that a laser or part of its beam train will fail, and need to be replaced, at any given time.
The time it takes to replace the laser on a laser-processing tool is “down-time” that increases ongoing production costs and may lead to costly missed delivery deadlines. Therefore, when a laser processing tool fails on a factory floor, it is highly desirable to replace the failed components quickly. For the reasons discussed above, the working laser and its beam-train optics are often the components most likely to fail.
However, most processing lasers and their optics cannot be quickly replaced when they fail because the unit-to-unit tolerances of these components exceed the alignment sensitivity of the process. The peak laser intensity, spot size, and position of the spot on the workpiece must almost always be tightly controlled. Some processes are also sensitive to the spot intensity profile, the shape of the wavefront at the workpiece, or the angle between the beam axis and the normal to the workpiece surface. Some of these key parameters (particularly spot size, spot intensity profile, and wavefront shape) are sensitive to changes in the optical path length (OPL) from the final component in the working-beam train to the workpiece. Surface contours on the workpiece, thermal alterations of workpiece thickness and other characteristics, thermal effects on the tool optics, and even thermal or pressure gradients in the atmosphere can change the OPL while a processing operation is ongoing. To ensure consistent performance after replacement of a laser or any of the beam-train optics, the system must be realigned before resuming use. Where multiple components are involved, realignment can take hours, and those hours of downtime add to production overhead costs.
The time, cost, and need frequency of alignments is further increased when laser-processing tools incorporate closed-loop control systems with active mechanisms to make “on-the-fly” adjustments during a processing cycle. Such control systems—for example, autofocus or leveling systems—are necessary to highly sensitive processes to compensate for variations in the characteristics of the working laser, the workpiece, or the surrounding environment. Sometimes the feedback for a closed-loop control system is an attenuated fraction of working laser itself, however, separate control beams are commonly used when the working laser is invisible, pulsed, or operating at a peak power that would damage readily available attenuators and detectors.
Depending on the sensitivity of the process to the parameter that the control beam controls, and on what type of light yields the best control data, control light sources may be low-power continuous-wave lasers, LEDs, or broadband lamps. Most control beams need to be aligned to run parallel or coaxial to the working beam they control. Some control beams, such as autofocus control beams, need to be calibrated to individual working beams when the laser-to-laser variations are too wide for the process to tolerate otherwise. The ability to calibrate control beams to a variety of individual working beams can relax tolerances on the expensive working lasers, reducing their cost. Therefore, replacement of failed processing lasers and beam-train optics must, in many cases, include recalibration of a separate control beam.
Quickly replaceable optical modules have been developed in such industries as fiber optic communications, printing, and information encoding and decoding. The modules include lasers and optics that have been pre-aligned on alignment fixtures and locked in place, and kinematic mounting features that precisely mate to corresponding features in the surrounding device. Commonly used kinematic features include spheres, sections of cones, rods, holes and slots, flats, and line contacts. However, the lasers in these modules are much lower in power, and both the lasers and the optics are both smaller in size and lighter in weight, than processing lasers and their optics. Many of the lasers incorporated into prior-art quickly replaceable modules also produce better beam quality and a smaller spot size over a longer distance, and therefore may have looser alignment tolerances in some cases, than higher-power multimode or superradiant lasers. Besides, although the prior-art modules often must maintain alignment over a wide range of ambient temperatures, the low-power lasers involved generally create very little heat of their own; nor are thermal effects on the workpiece usually a significant problem either. For these reasons, kinematic-module solutions that work well for low-power lasers cannot generally be easily adapted to high-power processing lasers.
Some pre-aligned kinematic modules have been devised specifically for processing lasers. In U.S. Pat. No. 5,748,827, Holl & Sabeti use a two-stage mount including a “macrostage” for coarse alignment, a “microstage” for fine alignment, and a compliant layer between the two stages. However, while their beam-positioning tolerance of ±10 μm is acceptable for their application of photocytometry and for some processing applications such as annealing, it is too loose for other applications such as holography and high-precision laser ablation.
In Published U.S. Pat. App. No. 2006/0249488, Jurgensen pre-aligns diode pump laser assemblies for fiber lasers that engrave ink-holding cavities on a metal printing drum. The working-spot size of 100 μm is too large for some ablation, micro-marking, and micro-bonding applications. Furthermore, while the '488 application's storage of spare lasers in place on the working laser support platform does reduce down-time, many factory environments, such as clean rooms where each cubic foot adds significant expense, cannot cost-effectively spare that much extra space on or around the tool for equipment that is not operating. Even where the space is available, ambient vibration, local shocks, or thermal cycling could gradually cause the pre-alignment to drift out of tolerance while the replacement lasers are stored on the platform.
In U.S. Pat. No. 6,424,670, Sukhman et al. pre-align laser modules and optics modules to be automatically interfaceable with each other when they are mounted on a common laser support platform. While '670 has an advantage of being able to replace either a laser module or an optics module, depending on where the failure occurs, it does not address thermal concerns—possibly because the '670 lasers seem to operate in enclosed cabinets whose temperature may be regulated—or situations where some separate element of a closed-loop control system needs to be calibrated to the individual replacement laser.
From the above discussion, none of the prior-art replaceable modules fully address the needs of high-power working lasers with separate control beams. Therefore, an unaddressed need exists for such a system.
When multiple working lasers operate together on a single tool, sometimes they all must be replaced at once. Sometimes two or more reach the end of their useful life at the same time. Also, in some settings such as job-shops where space is limited, budgets for expensive platforms and workpiece stages are tight, and processing needs change constantly, the ability to quickly exchange a group of processing lasers for a group with a different power, wavelength, or optical configuration type would be a significant economic advantage. Therefore, a largely unaddressed need exists for quickly replaceable groups of processing-laser modules that include aligned optics and calibrated control sources for each working laser in the group.